Laser beams have been successfully used as signal carriers for information communications systems for many purposes in recent years. For example, U.S. patent application Ser. No. 08/705,515 filed Aug. 29, 1996, for an invention entitled "Multiple Transmitter Laser Link", and assigned to the same assignee as the present invention, discloses a communication system which uses laser beam signal carriers. While laser beams can be very effective when used in a communications system, like all other types of communications systems, laser beam links are susceptible to disruptions. This can happen for a number of reasons. Perhaps the most obvious cause for the disruption of a laser beam is an interfering optical obstruction which is somehow positioned in the path of the laser beam. Such obstructions can include land based objects, such as buildings or mountains, or they may be airborne phenomena such as fog, clouds, smoke or haze. The land based obstructions can largely be avoided by proper placement of the laser communications link. On the other hand, while generally unavoidable, the airborne obstructions are usually temporary. It also happens, however, that less obvious disruptions to laser beams can occur due to atmospheric scintillations even though there are no optical obstructions per se.
Atmospheric scintillations can diminish the quality of a laser beam, and potentially disrupt communications carried on the beam, in several ways. For one, temperature gradients in the atmosphere will cause inhomogeneities in the index of refraction. Also, wind gusts can produce atmospheric scintillations which cause the intensity of the laser beam to vary erratically.
The ability of a laser link to handle atmospheric scintillations can, of course, be improved by using specially engineered equipment for the purpose. All equipment, however, has an operational threshold. In the specific case of laser beam communications equipment, there is an intensity of the laser beam below which the equipment is, for all intents and purposes, ineffective. It appears that this intensity threshold will exist no matter how sophisticated the equipment may be. Thus, whenever atmospheric scintillations cause the laser beam's intensity to fall below the equipment's operational threshold, there will be "drop outs" which cause so-called "burst errors" to occur in the communications. For example, if a laser transmitter transmits at the rate of 100 Mbits/sec (1 Mbit=10.sup.6 bits) a one msec (1 msec=10.sup.-3 seconds) drop out will lose 10.sup.5 bits of information. This loss rate is unacceptable.
For an effective communications link it is necessary that the drop outs, or burst errors, in a laser beam communications link occur less than 10.sup.-9 of the time. (e.g. At a data rate of 100 Mb/sec, it is acceptable to have a single bit error every 10 seconds on average.) Conventionally, in order to keep drop outs below this rate, the average operational intensity of the laser beam may need to be one hundred times the magnitude of the threshold intensity below which the equipment is operationally ineffective. To satisfy these requirements several solutions have been proposed. One solution is to upgrade the equipment that is used. For instance, equipment can be upgraded to have higher laser beam intensities. Also, upgraded equipment can employ larger receivers and/or multiple transmitters. Not surprisingly, the upgrading of equipment can be extremely costly. Another solution entails forwarding error correction codes along with the communications data. Several schemes can be envisioned for this solution. The problem, however, is that effective schemes for forwarding error correction along with the communicated data require a significant increase in memory and processor power. This too costs money, or may not be practical at very high speeds.
The present invention recognizes that while drop outs, if uncompensated, can be of sufficient duration to effectively disrupt communications (e.g. 1-10 msec), they occur for only a small percentage of the time (e.g. approximately less than 20%). Further, the present invention recognizes that the data to be transmitted over the laser beam can be effectively buffered to increase the rate of data transmission across the laser link and thereby allow the transmission of all data at times when there are no drop outs.
In light of the above it is an object of the present invention to provide a laser link for a communications system which transmits all communication data over the laser link only when there is sufficient quality in the laser beam to effectively transmit the data. Another object of the present invention is to provide a laser link for a communications system which accommodates disruptions in laser beam transmission that are due to atmospheric scintillations. Still another object of the present invention is to provide a laser link for a communications system which is relatively simple to manufacture, easy to use, and comparatively cost effective.